The present invention relates to a magnetic field apparatus for controlling the flow of molten steel in a casting mold, and more particularly to an apparatus for providing an adjustable magnetic field in a casting mold to impede and redirect in a controllable fashion the flow of liquid steel exiting from a submerged entry nozzle that discharges into the casting mold.
It is known in the art of steelmaking to continuously cast molten steel using an oscillating mold, typically a water-cooled copper-faced mold having a straight or curved channel. The mold typically has a rectangular horizontal cross-sectional forming conduit as thick and wide as the slab to be cast. Liquid steel in the upper portion of the mold is cooled as it moves downward through the water cooled mold, generating a steel shell as it passes through the mold before exiting the mold at the bottom. The molten steel enters the mold from a tundish through an entry nozzle submerged in the liquid steel in the mold. The submerged entry nozzle is normally located generally centrally of the mold cross-section, and is provided with opposed exit ports that direct liquid steel generally horizontally outwardly toward the narrow sides of the mold. Some nozzles have a bottom port as well.
The flow of liquid steel out of the submerged entry nozzle varies in direction and velocity due to various external conditions (such as the ferrostatic head of steel above the nozzle, and steel chemistry). This can create disturbances in the steel flow that adversely affect both the surface quality and internal quality of the casting. These disturbances tend to generate undesired temperature imbalances that interfere with uniform solidification of the steel as it passes through the mold and downstream thereof, and also increase the tendency of the steel to incorporate unwanted inclusions from the mold powder/slag/impurities mixture at the meniscus of the liquid steel at the top of the mold. A conventional magnetic brake inhibits these disturbances by reducing the velocity of liquid steel emanating from the submerged entry nozzle, thereby tending to constrict the eddies and prevent them from reaching the end edges of the mold and the upper surface of the pool of liquid steel at the top of the mold.
A conventional magnetic brake includes a magnetic circuit energized by direct or slowly varying electric current passing through windings around an iron core forming part of the magnetic circuit. The magnetic circuit passes through the wide faces of the mold so as to provide a magnetic field through the interior of the mold. Normally, in a conventional magnetic brake, the magnetic circuit passes through the mold about mid-way along the longitudinal length of the mold and overlaps the point of entry of liquid steel into the mold from the submerged entry nozzle, but does not extend up to the top of the liquid steel pool nor down to the bottom of the mold.
Although the magnetic field in a conventional magnetic brake can be varied (by varying the amount of current flowing through the windings around the iron core of the magnetic circuit) there is, nevertheless, typically no fine control over the manner in which the magnetic field is applied. Such fine control would improve the ability to control the flow characteristics of the steel as it exits from the submerged entry nozzle, in the interest of generating uniform solidification of the shell of cast steel emerging from the mold and in the interest of reducing unwanted inclusion and non-uniform surface effects.
Attempts have been made by various prior workers in the field to provide some variation in the magnetic field applied through the mold. Representative such attempts are disclosed, for example, in U.S. Pat. No. 5,404,933, issued Apr. 11, 1995 to Andersson et. al. (the Andersson patent), and U.S. Pat. No. 5,613,548 issued Mar. 25, 1997 to Streubel et. al. (the Streubel patent). The Andersson patent discloses an apparatus for controlling the flow of molten metal by applying a static or periodic low-frequency magnetic field across the area through which the molten metal flows. The Streubel Patent discloses an apparatus that accomplishes a similar result by attaching partial cores to a principal core surrounded by an electrical core, thereby influencing the magnetic field applied.
The present invention is directed generally to an apparatus for providing a magnetic field in molten steel inside a mold for casting molten steel, which magnetic field can be reconfigured so as to modify the flow characteristics of molten steel exiting from a submerged entry nozzle in the mold both by the use of (1) removable ferromagnetic or non-magnetic laminar elements positioned in the magnetic circuit adjacent the mold face, to accommodate changes in the chemistry and other characteristics of the steel to be cast in the mold, or (2) discrete individually energizable coils in the magnetic circuit during the casting of molten steel, in response to changing conditions in the molten steel, or both. It is contemplated that a suitable selection of ferromagnetic and non-magnetic laminar elements in a matrix array immediately adjacent the mold face will accommodate the more major and persistent changes in steel characteristics (e.g., steel chemistry), while the use of the individually engergizable coils (which may also be arranged in a matrix array adjacent the array of laminar elements) is intended to accommodate transient variations in the characteristics of the molten steel (e.g., ferrostatic head).
In the aspect of the present invention directed to providing a magnetic field that may be reconfigured between casting runs, there is provided a pair of magnetic poles comprising at least a pair of magnetic field cores, each core being energized by at least one discrete coil located in the vicinity of a discrete opposed wide face of the mold. The cores are connected by a yoke so that the cores and the yoke together with the mold containing molten steel form a complete magnetic circuit. When the coils are energized, the magnetic field extends generally horizontally from one wide face of the mold to the other. Each magnetic field core has one or more horizontal rows of generally horizontally disposed closely packed xe2x80x9cfingersxe2x80x9d in proximity to the proximate wide face of the casting mold. (The term xe2x80x9cfingersxe2x80x9d is used herein to identify a physically discrete projecting portion of the core adjacent the mold face, but it is to be understood that spaces between fingers is undesirable, although frequently necessary because of the need to accommodate opposed projections such as strengthening ribs on the surface of the mold.) The fingers protrude from the ends of their respective cores in two parallel, generally symmetrical generally horizontal arrays, each array abutting a respective face of the mold. (While the benefit of the invention as contemplated by the inventor is best obtained by having two generally identical matching arrays of fingers, one on either side of the mold, there may be circumstances in which the arrays are chosen not to be identical, or the fingers are provided on one side of the mold only.) The individual fingers in each array may abut one another, or some fingers may be slightly spaced apart so as to avoid interfering with other structural elements in the vicinity of the mold faces.
The fingers are comprised of removable ferromagnetic laminar elements and optionally spacers or non-magnetic laminar elements. These laminar elements for each finger are arrangeable in a vertically stacked array extending into proximity with the proximate wide mold face at a selected location. For continuity of the magnetic circuit, each finger should be positioned as close as possible to the adjacent mold face. The local magnetic field in the molten steel in the casting mold near each finger (each selected location) may be varied independently of the local magnetic field in the molten steel in the casting mold near the other selected locations by the addition or removal (effected between casting runs) of ferromagnetic or non-magnetic laminar elements to or from selected fingers, so as to modify flow characteristics of molten steel exiting from the submerged entry nozzle into the casting mold during casting runs. As it is desirable to have a generally uniform magnetic field across the entire transverse width of the array of fingers, fingers near the center of the array may have fewer ferromagnetic laminar elements attached than do fingers at the periphery of the array, to compensate for the natural tendency of the magnetic field to be stronger in the center. It may also be desirable to substitute non-magnetic laminar elements for ferromagnetic laminar elements in portions of the central fingers, or to provide spacers between selected successive ferromagnetic laminar elements, thereby creating air gaps in the magnetic field that serve essentially the same function as non-magnetic laminar elements.
To increase the degree of control of the magnetic field in the vertical direction, more than one horizontal array of fingers may be provided on each side of the mold face, or the capacity of each finger to accept laminar elements may be increased so that the vertical span of each finger is increased. If the first alternative is selected, additional rows of generally horizontally disposed closely packed fingers may be stacked vertically, creating a two-dimensional matrix of fingers, the amount and position of magnetic material in each finger being determined by selectively stacking ferromagnetic and nonmagnetic laminar elements. It may be desirable to provide an increased capacity to apply a magnetic field over the vertical dimension, such as by increasing the number of energizing coils and arranging them in a corresponding two-dimensional matrix, so as to accommodate any changes in the magnetic field distribution that the operator wishes to make.
Another aspect of the present invention is the provision of a magnetic field that may be reconfigured during casting. In this aspect, the magnetic field is created by a number of opposed pairs of magnetic field cores, each of which cores is energized by a discrete energizing coil. One core in each pair is located on one side of the wide face of the mold and its mating core on the other side of the mold directly opposite the first core. The terminal faces of each pair of opposed cores comprise poles of a component magnetic circuit, the overall magnetic circuit for the electromagnetic brake comprising the total of the component magnetic circuits. Each core is coupled within the magnetic circuit by an encircling yoke made from a magnetic material. A discrete individually controllable electrical current may be passed through each coil. When the mold contains molten steel, a composite magnetic circuit is formed, each component of which passes through one core of one discrete pair of cores, the yoke, the other core of that pair of cores, and the adjacent selected portion of the mold and the molten steel contained therein, so that when the coils are energized, the magnetic field extends from one wide face of the mold to the other. The local magnetic field in any one of the selected portions of the mold may be varied by varying the electrical currents passing through the pairs of coils associated with the pairs of magnetic field cores near that selected portion of the mold, so as to modify flow characteristics of molten steel exiting from the submerged entry nozzle into the casting mold. As each component magnetic circuit pole is provided with a discrete energizing coil, each pole pair may be energized independently of the other pole pairs, thereby providing control of the local magnetic field in the molten steel in the casting mold during casting.
In this further aspect of the invention each coil preferably energizes a portion of the core associated with at least one discrete finger having removable ferromagnetic and non-magnetic laminar elements. Note that the array of pole pairs and counterpart array of energizing coils may desirably correspond to the array of fingers, but need not do so.
The cores, including at least some of the removable ferromagnetic laminar elements, and the yoke should be made of iron or an alloy chiefly composed of iron. The removable non-magnetic laminar elements may be made of a heat resistant ceramic material. The ferromagnetic and non-magnetic laminar elements may be stackable rectangular parallelepiped plates, and they may be of varying heights and widths. If desired, some of the laminar elements may be dimensioned to span more than one finger.